The present invention generally relates to an optical disc device, and more particularly, to a focus draw-in method for an optical disc device.
It has been a recent trend that a large member of products related to optical discs such as compact disc players and the like, have been introduced into the market. Commonly, data of an optical disc are arranged in a spiral shape from an inner periphery to an outer periphery of the optical disc in the form of pits, and the data are settled thereon by the presence or absence of such pits. The role of an optical servo is to direct a focal point of light emitted form the optical device onto a predetermined row of pits during reproduction of data on the optical disc, thereby to establishing a state where the data is readily read out. The optical servo includes a focus control which functions to bring the optical device into focus on the disc face, and a tracking control for focusing onto the pit row on the disc face. For the control procedure, it is so arranged that, after rotating the disc by a motor, the optical device is driven in a direction perpendicular to the disc surface to effect the focus draw-in function.
For the explanation of the focus draw-in function, the formation of a focus error signal will be described hereinafter.
As shown in FIG. 8, the focus error signal becomes zero when the focal point is located on a disc face, while an absolute value of the focus error signal is increased as the focal point is spaced from the disc face in the perpendicular direction, and is reduced after reaching a peak value. In the focus control, it is so arranged that, when the focus error signal is present within an S-shaped peaks (i.e. within a range between peaks for an S-shaped curve: referred to as S-shaped peaks hereinafter), the absolute value of the focus error signal is reduced to zero for control so as to focus on the disc face, and the "focus draw-in" refers to the function at a stage prior to execution of the focus control.
Conventional focus draw-in systems are explained below.
Japanese Patent Publication Tokkosho No. 63-13261, issued on Mar. 24, 1988 to Minoru KOZAKE and Tetsuo SHIMIZU, and entitled "Focus servo device", and
Japanese Patent Publication Tokkosho no. 63-34528 issued on Jul. 11, 1988 to T. OKANO, and entitled "Focus lens servo draw-in device in optical information reading device" discloses two prior art focus draw-in systems.
In the focus servo device according to the above noted first prior art reference a reference voltage is produced with the same polarity as a focus error signal to be produced when the optical device is spaced from the disc beyond a "just focus" point, and the voltage has an absolute value smaller than that of the polarity value of said focus error signal, whereby when the focus error signal crosses the reference voltage at the side of the "just focus" point beyond the polarity value, the processing is transferred into the focus control.
Meanwhile, in the focus lens servo draw-in device according to the above noted second prior art, reference a reference voltage is produced with the same polarity as a focus error signal to be produced when the optical device approaches the disc beyond a "just focus" point, and the voltage has an absolute value smaller than that of the polarity value of said focus error signal, whereby when the focus error signal crosses the reference voltage at the side of the "just focus" point beyond the polarity value, a counter force gradually attenuated from a predetermined level is applied to the optical device, and when it crosses the reference voltage again, the processing is transferred into the focus control.
However, in the first prior art arrangement as described above, when the relative speed between the disc face and the optical device is large, it is impossible to stabilize within the S-shaped region of the focus error signal even upon transfer into the focus control, thus resulting in undesirable deviation from the S-shaped region.
Meanwhile, in the second prior art arrangement as referred to above, when the relative speed between the disc face and the optical device is large, it is also impossible to stabilize within the S-shaped region of the focus error signal, even if a counter force is applied, similarly resulting in deviation from the S-shaped region.